Scala Convert Array of Map of Maps to a Map With Lists of Maps

Question

I have an Array of Map of Maps and would like to merge the like keys and combine their values into a list. Is there an elegant way to do this, preferably without introducing any new libraries or updating scala? Scala version 2.11.12

(Map("a" -> Map(1 -> 1.1)),
Map("a" -> Map(2 -> 2.1)),
Map("b" -> Map(1 -> 1.1)),
Map("c" -> Map(1 -> 1.1)),
Map("c" -> Map(2 -> 2.2)))

Output:

Map(
"a" -> List(Map(1 -> 1.1), Map(2 -> 2.1)),
"b" -> List(Map(1 -> 1.1)),
"c" -> List(Map(1 -> 1.1), Map(2 -> 2.2)))

Edit to explain why it's not a duplicate of: Scala: Merge map That answer does not dynamically go through a whole list and merge identical keys, it only merges 2 specific maps given. As exemplified by the answer, the real solution to this has to do with an elegant way of iterating through the list and not only merging lists, but first loading the maps into a list and then merging them.

Why a List of Maps? Would not make more sense just merge all the inner maps in one map? - Or a List of tuples? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Sep 17, 2019 at 16:33
Not a duplicate, please take a look at selected answer. This is a single list with n objects, the link you've included is merging 2 maps — user3124181
– user3124181, Commented Sep 17, 2019 at 18:43

Pritam Kadam · Accepted Answer · 2019-09-17 18:36:50Z

4

you can try below


    val map = List(
      Map("a" -> Map(1 -> 1.1)),
      Map("a" -> Map(2 -> 2.1)),
      Map("b" -> Map(1 -> 1.1)),
      Map("c" -> Map(1 -> 1.1)),
      Map("c" -> Map(2 -> 2.2))
    )

    val merged = map.fold(Map.empty[String, List[Map[Int, Double]]]) { (a, b) =>
      a ++ b.map { case (k, v) => (k, (v :: a.get(k).toList.flatten).reverse) }
    }

    merged.foreach(println)

   /****** output ******
   (a,List(Map(1 -> 1.1), Map(2 -> 2.1)))
   (b,List(Map(1 -> 1.1)))
   (c,List(Map(1 -> 1.1), Map(2 -> 2.2)))
   *********************/

answered Sep 17, 2019 at 18:36

Pritam Kadam

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Comments

joesan · Accepted Answer · 2019-09-17 18:13:45Z

0

I would write my own function to do this. As a starter, something in the lines of:

type acc = Map[String, List[Map[Int, Int]]]

@tailrec
def merge(map: Map[String, Map[Int, Int]], acc): acc = map match {
  case Nil => acc
  case x :: xs =>
   val (key, value) = x
   if (acc.contains(key)) // append the value to the existing List and then call the merge method again
   else // create the new key and call the merge method again 
}

Make sure you use the new acc (the accumulator) each time you call the merge method!

answered Sep 17, 2019 at 18:13

joesan

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